A kind of automatic speed change mechanism for an automotive vehicle comprising a system which combines a torque converter equipped with a lock up clutch and an automatic transmission equipped with a forward/reverse change-over mechanism has, for example, been disclosed in Tokkai Hei 4-78369 published by the Japanese Patent Office in 1992.
In this speed change system, control of the speed changing of the automatic transmission, engagement and disengagement of the lock up clutch, and lubrication of the automatic transmission are performed by the use of pressurized hydraulic fluid from a single hydraulic pump. Hydraulic fluid which lubricates hydraulic transmission is supplied via a lubrication passage from a torque converter pressure passage which operates the lock up clutch.
Although this hydraulic pump may be constructed as a vane pump, the output pressure of such a pump is reduced when the temperature of the hydraulic fluid is high so that its viscosity is low, and also at times of low vehicle speed. In this connection, in order to ensure an adequate pressure for the engagement of the lock up clutch, an orifice is provided between the torque converter pressure passage and the lubrication passage.
On the other hand, when the automatic transmission is subject to high load, i.e. when the torque which is being transmitted by the automatic transmission is high, it is necessary to increase the amount of the lubrication hydraulic fluid for the automatic transmission.
In this case, although the output pressure of the hydraulic pump may be sufficiently high, the pressure in the torque converter pressure passage is controlled to a set pressure by a relief valve. As a result, even when the automatic transmission is being subjected to high load, it is difficult to make considerable increases to the flow of hydraulic fluid supplied from the torque converter pressure passage via the orifice to the lubrication passage.